Possibly they could end the first one in Mirkwood around the time the dwarves get caught by the spiders. Anywhere around there would make a good cliffhanger too. Though the entry to Mirkwood sounds like a good spot too.
I'm not exactly sure how much this is going to be stretched out. There is a wealth of background material in the Appendices. Perhaps they'll go into detail with Thror's ill-fated expedition to Moria and the war between the dwarves and the orcs, and perhaps give us some detail on Thrain's imprisonment. We might see some bits of the White Council debating on whetheror not to act against Sauron as the Necromancer in Dol Guldur. And there is the things that happen between the Hobbit and the Lord of the Rings that could be covered.
Also, some of the dwarves will probably get more time spent on them than was spent on them in the book. Only Thorin, Balin, Fili, Kili, and Bombur (and possibly Gloin) get much of any attention in the book, and here the dwarves will probably need to presented as more than just extras. That will probably stretch things out.
